A portable communication device has developed remarkably in recent years, and especially a cellular phone is being rapidly reduced in size, weight, and thickness. Other than the cellular phone, an electronic apparatus such as a video camera (handy camera or the like), a codeless telephone, or a lap-top personal computer (note type personal computer), is being reduced in size, weight, and thickness. The above can be used without being connected to a plug as a result that a secondary battery is mounted on an electronic apparatus main body, which increases portability and convenience. At present, the secondary battery is limited in capacity and it is necessary for the secondary battery to be charged once in several days to several weeks.
As a charging method, there are a contact charging method and a non-contact charging method. The contact charging method is a method in which an electrode of a power receiving device and an electrode of a power feeding device are made to contact directly and charging is performed. The contact charging method is generally used since its device structure is simple. However, as the electronic apparatus is reduced in size, weight, and thickness in recent years, the weight of the electronic apparatus becomes smaller, so that a contact pressure between the electrode of the power receiving device and the electrode of the power feeding device becomes insufficient, causing a problem that a charging defect occurs. Further, since the secondary battery is weak against heat, it is necessary to design a circuit so that excessive discharging or excessive charging does not occur, to prevent temperature rise of the battery. In view of the above, application of the non-contact charging method is being studied.
The non-contact charging method is a method in which both a power receiving device and a power feeding device are provided with coils and charging is performed by using electromagnetic induction. In the non-contact charging method, since it is not necessary to consider a contact pressure between electrodes, a charging voltage can be supplied stably without being influenced by a state of contact between the electrodes. As the coil for the non-contact charger, there are known a structure in which a coil is wound around a ferrite core, a structure in which a coil is mounted on a resin substrate where a ferrite powder or an amorphous powder is mixed, and so on. However, ferrite, becoming fragile if processed to be thin, has a problem of being weak in impact resistance and being apt to cause a defect in a power receiving device by dropping or the like of the apparatus.
In order to make a power receiving portion thinner to cope with decrease in thickness of an apparatus, it is studied to adopt a flat coil formed by printing a metal powder paste spirally on a substrate. However, a magnetic flux passing through the flat coil interlinks a substrate or the like inside the apparatus, there is a problem that an eddy current generated by electromagnetic induction causes heat generation in the apparatus. Thus, a large power cannot be transmitted and a charging time becomes long. Concretely, while it takes about 90 minutes for a contact charger to charge a cellular phone, it takes about 120 minutes for a non-contact charger to charge.
In a power receiving device to which a conventional non-contact charging method is applied, a measure against an eddy current generated by electromagnetic induction is not sufficient. Since the power receiving device has a secondary battery, it is required to suppress generation of heat to the utmost. Since the power receiving device is mounted on an electronic apparatus main body, generation of heat has a negative effect to a circuit component. Due to the above, a large power cannot be transmitted at a time of charging and a charging time becomes long. Further, generation of an eddy current leads to generation of a noise, which causes reduction of a charging efficiency. It is suggested, as a measure against the above, to provide a magnetic thin plate in a predetermined position of the power receiving device. By controlling a magnetic permeability and a plate thickness of the magnetic thin plate, or a saturation magnetic flux density and a plate thickness of the magnetic thin plate, heat generation by the eddy current, generation of the noise, reduction of the power receiving efficiency, and so on are suppressed.
A non-contact charging method is suggested in which a magnet is disposed in a power feeding side of a non-contact charger and positioning of an apparatus of a power receiving side is carried out. For example, in WPC (Wireless Power Consortium) being an international standard, a non-contact charger in which positioning is carried out by a magnet is described in “System Description Wireless Power Transfer, volume 1: Low Power Part 1: interface Definition version 1.0, July, 2010”.
When positioning is carried out by a magnet, magnetic saturation occurs in a conventional magnetic thin plate and a magnetic shield effect is substantially reduced. Thus, there is an apprehension that temperature rise of a secondary battery is brought about at a time of charging and that a cycle life time of the secondary battery is reduced. A conventional magnetic shield has a magnetic thin plate with a saturation magnetic flux density of 0.55 to 2 T (5.5 to 20 kG), for example, and such (a) magnetic thin plate (s), in a range of one to three or less, is (are) laminated. Even if a laminate of the magnetic thin plates is used as magnetic shield, there is a possibility that a magnetic field generated from a magnet disposed in a power feeding device easily causes magnetic saturation of the magnetic shied, a function as the magnetic shield not being exhibited.
In a present international standard of the non-contact charging method, there are a method of using a magnet and a method of not using a magnet in positioning of an apparatus in a power receiving side. Since a magnetic thin plate used in the conventional magnetic shield is excellent in soft magnetic characteristic, use of a laminate in a range of one or three or less magnetic thin plate(s) with a saturation magnetic flux density of 0.55 to 2 T causes magnetic saturation easily, if a magnet exists in the neighborhood. Under the circumstances, a magnetic sheet for non-contact power receiving device is desired which enables a sufficient magnetic shield effect and a high charging efficiency independently of existence/absence of a magnet in a power feeding device side.